1. Technical Field
The present invention pertains to a protocol to support Quality of Service (QoS) for point-to-multipoint voice conferencing in a mobile, Ad-Hoc wireless network. In particular, the present invention pertains to a scheme for conference call establishment, maintenance, and bandwidth reservation over IP networks that allows for half-duplex communication between a large number of call participants organized in a self-forming network supporting integrated voice/data traffic.
2. Discussion of the Related Art
Traditional cellular wireless network systems employ fixed base stations interconnected by a wired infrastructure in order to enable communication between mobile users within one hop of a corresponding base station. However, tactical communication systems must permit armed forces and their mobile platforms to communicate and move about freely without the restrictions imposed by wired communication devices. Further, a communications system for coordinating tactical operations functions in a distributed manner is required to avoid centralized control points that render the network vulnerable in the event of a failure.
In order to address the limitations of the cellular model, new applications have emerged that allow multi-hop communication between remote users without relying on the fixed infrastructure. For example, a system developed for the Multimedia Mobile Wireless Network (MMWN) project transports multimedia traffic over mobile, Ad-Hoc tactical networks. MMWN is based on a hierarchical network architecture and uses a set of link and network layer algorithms to support distributed real-time multimedia applications in a mobile, Ad-Hoc network. The MMWN system is based on a link-state routing distribution mechanism.
MMWN has the following three components: clustering techniques, location management, and virtual circuit setup and repair. The system constructs and maintains Elastic Virtual Circuits (EVC) to transport multimedia traffic. These circuits may include multiple branches to reach multiple call participants, are designed to adapt to node mobility, and allow for call participants to randomly leave and join the call throughout the lifetime of the circuit. Virtual circuits are formed on a hop-by-hop basis, where hop-by-hop routing is based on a distributed computation of the forwarding path. Each node along the circuit performs a Dijkstra calculation to determine the next appropriate hop. The circuits provide quality-of-service (QoS) routing, resource reservation and loop elimination in the presence of inconsistent routing information. These circuits further include a repair mechanism in case of node movement or failure. Maintenance and repair of the virtual circuit is based on link-state information and requires the transmission of control information.
A mobile, Ad-Hoc wireless network operates in a highly dynamic environment. Due to node mobility and multi-path and channel fading, routes and resource availability fluctuate rapidly at wireless nodes. State information used by traditional routing protocols is quickly rendered obsolete because the nodes move rapidly. Thus, routes frequently become disabled using traditional routing protocols and are continuously re-computed. This results in an interruption in data connection and a loss of quality at the application. Therefore, the QoS provisioning problem in wireless networks is significantly more challenging than in wired networks.
The virtual circuit setup scheme described above may not be suitable in a highly dynamic, bandwidth restricted environment because of the time and bandwidth required in setting up and maintaining the virtual circuit. In order to set up and maintain a virtual circuit and reserve resources, nodes along the circuit must transmit control messages issued by the source and sent toward call participants, where the call participants must respond to these messages. In scenarios with rapidly changing topology, the time interval required for the circuit to adapt to a topological change may exceed the time interval over which changes occur and, hence, exceed the time interval in which the protocol can adapt to the changes. Subsequently, routing tables may not be able to converge. In a highly volatile situation, this hop-by-hop routing mechanism tends to lead to inefficient routes, excessive bandwidth consumption and an inability to locate call participants.
The conference call service of the present invention extends the principles of a cellular system to a mobile, multi-hop Ad-hoc environment (e.g., Small Unit Operations (SUO)). Multimedia applications (e.g., digital audio and video) have stringent QoS requirements due to the timeliness of the information. In traditional single hop cellular networks, nodes learn about resource allocations via a base station. The present invention extends this solution to the multi-hop, Ad-Hoc environment, where the bandwidth required to support voice calls is reserved via a TDMA scheme. This guarantees the bandwidth for real-time traffic.
The present invention differs from the MMWN approaches described above with respect to at least the resource reservation scheme, call signaling procedure, and dissemination of voice packets through the network. The broadcast nature of a wireless medium provides opportunities for efficient dissemination of multi-destination voice packets. The present invention employs a TDMA reservation and broadcasting scheme. This type of scheme is not usually proposed for mobile, Ad-Hoc systems since the scheme is considered to consume excessive bandwidth. However, under highly mobile conditions, the omni-directional broadcast consumes minimal transmission resources at the transmitting node, incurs minimal delay for each hop, and is simple to implement. In the present invention scheme, no overhead associated with repair or maintenance of a circuit is necessary.
Further, wireless medium access control (MAC) protocols may cause a network to collapse due to the congestion that can result from control overhead (e.g., request to send (RTS)/clear to send (CTS)/acknowledgement (ACK)), wasted capacity and the retransmission of failed packets due to collisions, interference and blocking. A collision occurs when a node within range of a destination attempts to access a channel while receiving a packet. The RTS/CTS handshake can be very inefficient and the overhead associated with access schedules further decreases capacity. In order to avoid collapse of the networking communication system due to congestion, the broadcast and dynamic TDMA reservation approach of the present invention avoids the RTS/CTS overhead associated with scheduled access protocols by eliminating the need for contention.
In a mobile and volatile environment of a tactical communication system, traditional routing schemes can breakdown due to an inability to track a destination and the excessive bandwidth expended in the process. Since routing on a down-link from a base station to call participants is not required and since flooding (e.g., consuming valuable resources) is not employed, the present invention is suited for a dynamic, volatile environment.